The present invention relates to alkaline earth thioaluminate phosphor materials that contain gadolinium as a co-activator. In particular, the present invention relates to the use of gadolinium as co-activator for such phosphors when deposited as thin films in electroluminescent displays. The present invention also relates to improving the luminance of phosphor materials used for full colour ac electroluminescent displays, particularly those employing thick film dielectric layers with a high dielectric constant.
Thick film dielectric structures provide for superior resistance to dielectric breakdown, as well as a reduced operating voltage, compared to thin film electroluminescent (TFEL) displays e.g. as exemplified by U.S. Pat. No. 5,432,015. The thick film dielectric structure when it is deposited on a ceramic substrate will withstand higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. With these advantages and with recent advances in blue-emitting phosphor materials, displays have approached the luminosity and colour coordinates required to achieve the technical performance of traditional cathode ray tube (CRT) displays. Nevertheless, further improvement in green and blue phosphor performance is required to simplify display design, to improve display reliability by lowering operating voltages and to keep pace with a trend towards higher colour temperature specifications for displays.
Traditionally, the phosphor materials of choice for full colour electroluminescent displays have been cerium-activated strontium sulphide for blue and manganese-activated zinc sulphide for red and green colours. The optical emission from these phosphor materials must be passed through an appropriate chromatic filter to achieve the necessary colour coordinates for red, green and blue sub-pixels, resulting in a loss of luminance and energy efficiency. The manganese-activated zinc sulphide phosphor has a relatively high electrical to optical energy conversion efficiency of up to about 10 lumens per watt of input power. Cerium-activated strontium sulphide phosphor has an energy conversion efficiency of 1 lumen per watt, which is relatively high for blue emission. However, the spectral emission for these phosphors is quite wide, with spectral emission for the zinc sulphide-based phosphor material spanning the colour spectrum from green to red and that for the strontium sulphide-based material spanning the range from blue to green. This necessitates the use of the optical filters. The spectral emission of the cerium-activated strontium sulphide phosphor can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, but not to the extent required to eliminate the need for an optical filter.
Alternate blue phosphor materials that have narrower emission spectra tuned to provide the colour coordinates required for a blue sub-pixel have been evaluated. The materials include cerium-activated alkaline earth thiogallate compounds, which give good blue colour coordinates, but have relatively poor luminosity and stability. Higher luminosity and excellent colour coordinates for blue pixels have been achieved with europium-activated barium thioaluminate phosphor materials. Higher luminosity and excellent colour coordinates for green pixels have been achieved with europium-activated calcium thioaluminate phosphor materials.
The use of gadolinium as a co-activator to enhance the luminosity of rare earth-activated zinc sulphide phosphor materials in thin film electroluminescent devices is known. In particular, U.S. Pat. No. 4,967,251 teaches the use of a red light emitting samarium-activated zinc sulphide phosphor layer in a thin film electroluminescent device. Co-doping of the phosphor material with less than 2 atomic percent of gadolinium resulted in the luminosity being increased by about two-fold. Use of a gadolinium co-activated thulium-activated zinc sulphide phosphor for blue light emission was also described. However, even with the co-activator, the blue luminance was insufficient for use in commercial electroluminescent displays.
Gadolinium is known to function as an activator for the emission of ultraviolet light in a zinc magnesium sulphide phosphor material. For instance, U.S. Pat. No. 5,670,839 teaches an electroluminescent device with a gadolinium-activated Zn1-xMgxS phosphor layer where 0.33 less than x less than 1. The phosphor emits ultraviolet light with a wavelength of 310 nanometers. The zinc magnesium sulphide material is said to have a sufficiently large band gap that it does not absorb the ultraviolet light generated by the gadolinium activator. Use of such an ultraviolet emitting phosphor in conjunction with a second adjacent phosphor film that can be photostimulated by the ultraviolet light to emit visible light and thereby create a display is also disclosed. The visible light emitting activators were in a different material from the gadolinium activators.
One aspect of the present invention provides a thin film phosphor for an electroluminescent device, said phosphor being selected from the group consisting of thioaluminates, thiogallates and thioindates having at least one cation selected from elements of Groups IIA and IIB of the Periodic Table of Elements, said phosphor being activated by a rare earth metal and co-activated with gadolinium.
In a preferred embodiment of the present invention, the phosphor is a thioaluminate.
In another embodiment, said rare earth metal is europium or cerium, especially europium.
In another preferred embodiment, the cation is calcium.
In further embodiments, the phosphor contains europium in an amount in the range of 1 to 10 atomic percent and gadolinium in an amount in the range of 1 to 5 atomic percent. Preferably, the amount of europium is between two and eight percent and the amount of gadolinium is between two and four atomic percent.
Aspects of the present invention also provide an electroluminescent device comprising a thin film phosphor as described herein on a substrate.
Further aspects of the present invention provide an electroluminescent device in which the thin film phosphor is adjacent to a thin film of zinc sulphide. Preferably, the thin film phosphor is sandwiched between thin films of zinc sulphide.